Okay, so companies like GMD and DiaperRite just took standard sized prefolds like yours and started making them shorter so parents did not have to fold them down, and reduce bulk. So all you need to do is serge a couple inches off one end. No anyone witth a serger? I think you want them about 15 inches long (I am assuming they are about 13" wide when shrunk). That would make them the right size.

Also, it works great to trifold your smaller ones to make them last longer. I could easily use 10"x12" prefolds trifolded the long way until my kids were 18 pounds